This invention relates to the repair of articles coated with a metallic environmental resistant overlay type coating of an alloy including aluminum. More particularly, it relates to a method for repairing a surface connected fissure in an article, for example a turbine engine component coated with an overlay coating.
Surfaces of certain components of gas turbine engines operating at relatively high temperatures in oxidizing and corrosive conditions generally are protected with an environmental resistant coating or coating system. Such components, made of a high temperature superalloy based on at least one of Fe, Co, or Ni, include blading type members, for example turbine blades, vanes, nozzles and/or struts, as well as shrouds, which include a coated surface. A frequently used and widely reported commercially available type of metallic environmental resistant coating including the element Al, and sometimes referred to as an overlay coating, is the Mxe2x80x94Al or Mxe2x80x94Crxe2x80x94Al base coating. In that type of coating, xe2x80x9cMxe2x80x9d is Fe, Co, Ni, or their combinations. In some forms, the coating can include one or more additional elements such as Y, Hf, rare earth elements, etc. Such an overlay coating is described in many U.S. Patents including U.S. Pat. No. 4,117,179xe2x80x94Jackson et al. (patented Sep. 26, 1978).
Such metallic overlay type of coating differs from the frequently used and widely reported diffusion aluminide type of coating, for example the commercially available Codep aluminide coating, in the chemical compositions of their outer portions as well as of their diffusion zones generated beneath such coatings. One major difference is in the amount of Al in the respective diffusion layer or zone generated beneath the coatings. For example, about 30-35 wt. % Al can be found beneath an outer or additive portion of a diffusion aluminide coating compared with about 6-15 wt. % Al beneath an overlay coating.
As a result of service operation under such strenuous conditions in a gas turbine engine, fissures, sometimes referred to as cracks, breaks, or openings, have been observed in the outer surface of the above described type of metallic overlay environmental resistant coating. Such a fissure has been observed to extend from the overlay coating outer surface generally inwardly through such coating and toward a substrate of an article. This exposes the overlay coating at its surface within the fissure. Because such components of gas turbine engines are costly to manufacture, it is preferable to repair rather than replace the component, even if the fissure extends into the substrate and requires repair such as welding or brazing.
Some currently used repair methods include complete removal of the metallic environmental resistant coating, for example by mechanical means such as through use of abrasive materials or chemical means such as aqueous acid solutions, typically a mixture of nitric and phosphoric acids. One reason such removal has been conducted is that the presence of Al in the surface to be repaired has been observed to be detrimental to repair methods including at least one of welding, brazing and application of a replacement coating. However, such methods of stripping a metallic coating, particularly one that includes an inner diffusion zone, can result in removal of substrate material to an amount below a dimension requirement of the article. For example, such removal of a metallic environmental resistant coating from the outer wall of an air cooled blading member can result in reduction in wall thickness to below allowable design thickness and/or increase in the size of surface connected airflow openings, detrimentally changing cooling airflow.
In another method described in U.S. Pat. No. 5,728,227xe2x80x94Reverman, Al is depleted from a coating inner portion of a coating diffused with a substrate. The inner diffused portion is exposed by mechanically removing a coating outer portion to enable Al to be depleted from the coating inner portion.
The present invention, in one form, provides a method for repairing an article including a metallic overlay environmental resistant alloy coating including Al without removing the coating. The coating, which has a coating thickness, has therein a fissure extending from a coating outer surface generally toward an article substrate, the coating having an exposed coating surface within the fissure. The method includes depleting an amount of more than about 90 wt. % of the Al from the exposed coating surface within the fissure and from the outer surface of the coating at least adjacent the fissure. Such Al is depleted to a depth of at least about 7 mils from the exposed coating surface within the fissure and from the coating outer surface, without removal of the coating and while substantially maintaining the coating thickness.
After such preparation, the fissure is repaired.